Spontaneous activity in developing sensory systems has been shown to be important for the growth and survival of projection neurons and may help to refine and stabilize sensory maps in the brain. Recent in vivo studies indicate that bursts of action potentials occur in afferent spiral ganglion neurons prior to the onset of hearing;however the mechanisms responsible for generating this activity have not been determined. Our preliminary studies indicate that inner hair cells (IHCs) in the developing cochlea are periodically depolarized by ATP that is released spontaneously from neighboring supporting cells. These ATP-dependent depolarizations can trigger Ca2+ action potentials in IHCs, suggesting that extracellular ATP may be responsible for initiating activity in auditory nerve fibers. Here, we propose to test the hypothesis that `spontaneous'activity in primary afferent spiral ganglion neurons prior to the onset of hearing requires purinergic receptor activation. We will use cochlear explant cultures prepared from prehearing rats to characterize the patterns of spontaneous activity that occur in spiral ganglion neurons, and determine whether this activity is dependent on ATP receptors and requires gap junctions/hemichannels. In addition, we will examine the firing patterns of spiral ganglion neurons in vivo, by making extracellular recordings from these neurons in prehearing rats. To test whether the in vivo firing of spiral ganglion neurons is mediated by ATP, we will infuse P2 receptor antagonists into the cochlea while recording extracellular action potentials from these neurons. These experiments seek to define the patterns of activity carried by auditory nerve fibers during the prehearing period and determine if this activity is mediated by the release of ATP within the cochlea. Ultimately, the data gained from these mechanistic studies will enable us manipulate this activity in vivo to determine the specific role that activity plays in auditory system development. PUBLIC HEALTH RELEVANCE: This proposal seeks to determine the mechanisms responsible for initiating sensory-independent activity in auditory nerves before the onset of hearing. As our preliminary findings suggest that ATP release from supporting cells may play an important role in the initiation of this activity, these studies may yield new insight into how mutations in gap junctions and other supporting cell-associated genes result in deafness. Furthermore, our experiments may have direct relevance to human conditions where sound in perceived in the absence of sensory input, such as peripheral tinnitus.